Method for detecting failed printing nozzles in inkjet printing systems and inkjet printing machine

ABSTRACT

A method for automated detection of failed printing nozzles in an inkjet printing machine includes the following steps:
         carrying out a printing process to produce a first printed image with print image data,   scanning and digitizing the first printed image produced in the printing process by using at least one image sensor,   digitally offsetting the first printed image in the control unit by at least one printing nozzle in a direction transverse to the printing direction,   carrying out a further printing process to produce a second printed image based on the digitally offset first printed image,   scanning and digitizing the second printed image by using the at least one image sensor,   comparing the first and second scanned printed images in the control unit.   Failed printing nozzles are identified by using the control unit based on the result of the comparison.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority, under 35 U.S.C. §119, of GermanPatent Application DE 10 2015 207 566.4, filed Apr. 24, 2015; the priorapplication is herewith incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a method for the automated detection offailed printing nozzles in an inkjet printing machine by using a controlunit. The present invention also relates to an inkjet printing machinehaving a control unit for implementing the method.

The technical field of the invention is the field of digital printing.

Inkjet printing machines in general include one or more print heads andevery print head includes a plurality of printing nozzles. The inkjetprinting machines use the nozzles to print by the ejection of ink. Theprinting machines have nozzle plates with specific configurations of theindividual nozzles, allowing a resolution of up to 1200 dpi. Thatrequires nozzle interspaces of approximately 20 μm. When an individualnozzle fails, there are areas that cannot be imaged by the nozzle thatwas provided for that purpose in the individual color separation inaccordance with BCMY. As a result, colorless areas are created, whichmay occur as white lines. In a multicolor print, the corresponding coloris missing at the point in question and the color values are distorted.Another aspect is that the ejection path of an individual nozzle is notideal but may deviate from the ideal path to a greater or lesser extent.In addition, the size of the jetted dot is to be taken intoconsideration. Thus a malfunctioning nozzle has an effect on the printquality of every printed document. The reasons for such a failure ofindividual nozzles are manifold. The failure may be temporary orpermanent.

Various compensatory approaches are known in the art in order to reducethe effect on the printed image in solid areas in particular. In oneapproach, an attempt is made to cover up the defect by other nozzles ofthe same color and of the same inkjet unit. That is to say that tocompensate for individual failed inkjet printing nozzles, once theindividual nozzle has been identified, the adjacent nozzles arecontrolled in such a way that the dot sizes of those nozzles areincreased to such an extent that they also cover the area of the failednozzle. Thus the adjacent nozzles write the image of the failed nozzle.White lines that occur when individual nozzles do not print may thus beavoided.

Another known approach is to replace the failed printing nozzle by thenozzles of the respective other printing colors in use at the samelocation. In that process, an attempt is made to get as close aspossible to the failed printing color by a systematic and controlledoverprinting of the colors that are still available. That does notrequire a redundancy of printing nozzles or printing heads nor does afailure of adjacent nozzles present a problem. A major disadvantage ofthat compensatory process is, however, that it can only be used formulticolor printing. Moreover, it requires an increased computing andcontrolling effort by the control unit of the printing machine toestablish the required color combinations. In addition, the print resultmay well deviate significantly from the target values depending on thecolor difference of the failed color from the still printable colorspace of the remaining colors.

Other approaches to compensate for failed printing nozzles envisage theprovision of double nozzle units of the same color to be able tocompensate the failure of individual nozzles by redundancy.Alternatively, multiple positionable print heads are used to print animage. If printing nozzles fail, the print heads are repositioned toreplace the failed nozzle as well as possible. Both approaches de factorequire a redundancy of print heads of the same color, involving theaforementioned problems.

A prerequisite for such a compensatory process is, however, the correctdetection of a failed printing nozzle, involving not only the detectionof the failure itself but also the identification of the actual failednozzle because most known compensatory processes require the exactknowledge of the non-functioning printing nozzles.

Several approaches to a solution of this detection problem are known inthe art:

1. Printing Test Prints:

The test prints are then evaluated by the machine operator, i.e. countsare made. The information on potentially failed nozzles is forwarded tothe machine by a manual input. Based on that information, a new printedimage is created in such a way as to compensate for the failed nozzles.That process may not be carried out during primary processing time. Adefect in a printed image needs to be detected first to subsequentlyinitiate the manual process described above. An inspection is necessary,resulting in a loss of production time. In addition, there is noautomatic detection and in some cases waste may be the result. Examplesof such sample prints are known from U.S. Patent Application PublicationUS 2011/227988 A1 and U.S. Pat. No. 8,322,814 B2.

2. Printing and Automatically Evaluating a Specific Test Content:

Option A: the test content is printed as a separate job on the printingmachine.

Option B: the test content is printed between the individual copies in aweb-fed printing process or on an unused paper margin in a sheet-fedprinting process.

That test content provides a comparatively simple automated detection ofthe failed inkjet nozzles. A disadvantage is that a paper margin orinterspace between individual copies is undesirable or cannot beimplemented for some types of prints. If the sample print is created asa separate print job, a lot of waste is created. A narrow paper marginonly allows test contents of limited size, allowing only part of thenozzles to be inspected. That means that an immediate detection andcompensation of failed nozzles is not guaranteed. Waste may be producedor alternatively, the paper format may be used insufficiently.

3. Other Options:

Option A: the entire printed image is scanned in real time by usingcameras or sensors. The established data then need to be electronicallycompared to the original printed image. However, the comparison of thedata requires a very high computational effort and real-time comparisonsbetween the data. When variable data are used, that means that for everyprint, the target printed image needs to be made available again forcomparison or adapted in accordance with the variable data. Thatsolution to the problem is very costly since it requireshigh-performance hardware or creates machine downtime while the data arebeing processed. The system is prone to errors because it is notimmediately clear exactly which nozzle row has failed in order to thenmake compensations. Electronic measuring would require high-precisionequipment and would be very costly.

U.S. Patent Application Publication US 2013/187970 A1 is to be cited asan example of such a process. In that case, the digital target image iscompared to the scan of the printed image. Transformations that make thescanned image (resolution transformation, transformation of the scannercharacteristics) comparable to the digital target image are described.In addition, the document describes the calculation of a difference thatis used for detecting a non-functioning nozzle when deviations exceedingdefined thresholds occur. The document also mentions printing areference mark through the use of which a positiondetection/identification of the non-functioning printing nozzle may beachieved.

Option B: in this case, the entire printed image is likewise scanned inreal time by using cameras or sensors. However, then the data aredigitally added up in the printing direction in terms of the grayvalues/intensities or similar variables and a profile transverse to theprinting direction is established. If that profile has pixel-wide“drops,” the conclusion is that a malfunction has occurred. A majordisadvantage in that context is that an intended drop, for instance whenprinting a bar code, cannot be differentiated from a nozzle malfunction.Known examples from the prior art include U.S. Pat. No. 8,531,743 B2,which describes a system for detecting failed nozzles wherein an imagerecorded by an optical sensor is searched for strips of differentintensity along the printing direction. In that process, an integratedprofile is created, searching for drops that drop below a threshold. Thedocument explains methods that allow the scans/lines recorded by thesensor to be allocated to individual printing process colors in order todetect the respective failed nozzles in the printing color.

Another example is European Patent Application EP 2 626 209 A1, whichlikewise envisages a detection of strips of different intensity alongthe printing direction. In that process, changing light, i.e. light ofdifferent wavelengths, is used to create a respective image per processcolor, which is then directly searched for strips in the scanned colorseparation. In that process, an integrated profile is created, which issearched for drops that drop below a threshold.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method fordetecting failed printing nozzles in inkjet printing systems and aninkjet printing machine, which overcome the hereinafore-mentioneddisadvantages of the heretofore-known methods and machines of thisgeneral type in terms of the necessity of a sample print and lowperformance.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a method for the automated detection offailed printing nozzles in an inkjet printing machine using a controlunit, which comprises the following steps:

-   -   1. Carrying out a printing process to produce a first printed        image based on print image data,    -   2. Scanning and digitizing the first printed image produced in        the printing process by using at least one image sensor,    -   3. Digitally offsetting the first printed image by at least one        printing nozzle in a direction transverse to the printing        direction in the control unit,    -   4. Carrying out a further printing process to produce a second        printed image on the basis of the digitally offset first printed        image,    -   5. Scanning and digitizing the second printed image by using the        at least one image sensor,    -   6. Comparing the first and second scanned printed images in the        control unit, and    -   7. Identifying failed printing nozzles based on the result of        the comparison by using the control unit.

The basis for the method of the invention is the detection of failedprinting nozzles by offsetting the printed image. For this purpose, thefirst printed copy is scanned by a camera and offset in the control unitby at least one printing nozzle in a direction transverse to theprinting direction. Whether the offsetting is done to the left or to theright is irrelevant as long as there are enough so far unused nozzles inthe offsetting direction for the entire copy to be printed despite theoffset. Once the printed image has been offset, the next copy isprinted, digitized once again, and compared to the old, non-offset imagein the control unit. Failed printing nozzles may be detected because inthe second copy, vertical free areas in the printing direction have“wandered” by the offsetting width in the second image. If they wereintegral parts of the printed image, they would have to appear at thesame image location in the offset printed image.

An advantage of this method over the known methods of the prior art isthat it does not require any specific sample print for detecting anozzle failure because the detection is based on the actual printedimage.

In contrast to the known methods that implement the detection of anozzle failure in the printed image, however, no knowledge of a targetprinted image is required. In contrast to known methods that identifymissing nozzles in the printed image without knowing the target printedimage, no complex analyses of the printed lines over many prints arerequired. In accordance with the invention, a genuine differencerecognizable to an image sensor between two copies of a printed image iscreated, the sole cause of which may be a defective printing nozzle.Thus the detection obtains a much higher degree of reliability; inaddition, the time required for the inspection is much shorter since thenumber of required printed copies is lower.

Advantageous and thus preferred further developments of the inventionwill become apparent from the discussion below and from the descriptionwith the associated drawings.

In this context, a preferred further development is that in addition tooffsetting the first digital printed image by at least one printingnozzle in a direction transverse to the printing direction, the printhead is offset by the same amount in the opposite direction.

Since the control unit needs to deduct the offset of the printed imageprior to the comparison between the first and second digitizedcopy—after all, the image sensor continues to be in the same location—itis expedient to compensate for the merely digital offset of the printedimage by mechanically offsetting the print head by the same amount inthe opposite direction. Thus for the image sensor, the copy is in theold location whereas the offset of the printed image to adjacent nozzlesis maintained. This reduces the computing effort involved in thedetection for the control unit. Alternatively, the image sensor may bemoved by the same amount and in the same direction as the digital offsetof the printed image.

In this context, another preferred further development is that thecomparison between the first and second scanned digital printed imagesis done by calculating the difference.

The easiest and most efficient way to compare the two scanned copies isto calculate the difference. A relevant color value may only be presentin locations of a defective printing nozzle.

In this context, a preferred further development is that the result ofthe detection is output to an operator by the control unit on a display.

Although the detection and compensation will preferably be carried outin an automated process in the framework of a workflow process for theprinting process, an optional feedback of the detection result to thehuman operator on a graphical display is a necessary part of the methodof the invention.

In this context, another preferred further development is that thedetection result is used as a trigger for the initiation of acompensation mode of the inkjet printing machine for the at least onefailed printing nozzle.

As mentioned above, the detection result is used as a trigger for theactivation of a compensation mode in the framework of the workflowprocess.

In this context, a preferred further development is that the imagesensor only scans an image section of the respective printed image.

The detection process does not have to be carried out over the entireprinted copy. It is sufficient to scan only one strip that includes allactive printing nozzles. Even smaller inspection areas are possiblealthough they will necessarily mean a corresponding loss of information.

In this context, another preferred further development is that thescanned image section is added up during the ongoing printing process tocreate a brightness profile and the difference is calculated between thebrightness profiles of the scanned image sections of the respectivefirst and second digital printed images.

Since the image artifacts caused by the failed printing nozzlesnaturally occur in the shape of stripes in the printing direction,instead of a comparison over the entire scanned copy/inspection area, itmay be added up to create a brightness profile and then the comparisonmay be made between the two brightness profiles. This means a massivereduction of the computational effort for the control unit.

In this context, a preferred further development is that the firstdigital printed image is offset by at least two printing nozzles in adirection transverse to the printing direction and in increments of atleast one printing nozzle.

The offsetting of the digital printed image by the total offset mayoccur in a number of increments. For this purpose, a total offset by thedistance of at least two printing nozzles is required. The result is an“optical flow” in the offsetting direction with the exception of thoselocations in which there are defective printing nozzles. This approachmay for instance be of advantage if the printed copy contains imageelements that are similar to the stripe-shaped image artifacts or iflarger completely unprinted areas are present in the printing direction.

With the objects of the invention in view, there is concomitantlyprovided an inkjet printing machine for implementation of the method toattain the proposed object of the invention.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a method for detecting failed printing nozzles in inkjet printingsystems and an inkjet printing machine, it is nevertheless not intendedto be limited to the details shown, since various modifications andstructural changes may be made therein without departing from the spiritof the invention and within the scope and range of equivalents of theclaims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a longitudinal-sectional view of an example of a web-fedinkjet printing machine;

FIG. 2 is a plan view of an example of an image defect created by aprinting nozzle failure;

FIG. 3 is a block diagram illustrating the construction of the printingmachine system being used;

FIG. 4 is a diagrammatic representation of a detection process; and

FIG. 5 is a flow chart of the method of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now in detail to the figures of the drawings, in whichmutually corresponding elements have the same reference numerals, andfirst, particularly, to FIG. 1 thereof, there is seen a preferredembodiment in which the area of application is an inkjet printingmachine 1. An example of the construction of such a machine 1 is shownin FIG. 1. The inkjet printing machine 1 includes an unwinding unit 2from which a web is unwound and fed to a print preparation stage 3having a flexographic unit 4 for white/solid areas and a flexographicunit 5 for primer. The web is then fed to a printing unit 6 having printheads 7 with nozzles. The web subsequently travels to a flexographicunit 8 for varnish in a further processing unit 9. Finally, the web iswound up in a wind-up unit 10.

As described in the introduction, individual printing nozzles in theprint heads 7 in the printing unit 6 may fail during operation of theprinting machine 1. As a consequence, white lines 13 or, in the case ofa multicolor print, distorted color values in a copy 12 on a printingsubstrate 11 will occur. An example of such a white line 13 is shown inFIG. 2.

Since a manual implementation of the described method by an operatorwould be inefficient, the method is carried out in an automated way by acontrol unit 15 of the inkjet printing machine 1. FIG. 3 illustrates anexample of the construction of such a system. The automated method isintegrated in the workflow of the printing machine 1. The configurationof the control unit 15 in terms of individual method steps may bemanually corrected by the operator if necessary. The control unit 15 ispart of a printing machine control 14.

The functional principle of the detection method is shown in detail inFIG. 4 in the form of a preferred exemplary embodiment. Every printedimage or a section 20 of an image is scanned by an image sensor. Beforethe next copy or section 20 of the image is printed, the printed imageis electronically offset by one or more printing nozzles in a directiontransverse to the printing direction, causing different nozzles to printthe image information. Simultaneously, a mechanical adjustment of theprint heads counter to the electronic offset by the same amount and in adirection transverse to the printing direction is implemented, causingthe next copy of the printed image to be printed in the same positionfrom the point of view of the image sensor. In terms of the imagesensor, the opposite offsets of the digital printed image and of thephysical position of the print head cancel each other out. This processchanges the assignment of the lines/columns of the image to the nozzlesby the amount of the digital offset. In the resultant image, which hasbeen offset twice in opposite directions, a line-shaped artifact that iscreated by a printing column without printing data, i.e. a desired lineis maintained in its original position. In contrast, a line-shapedartifact 13 in the form of a printing column 21 caused by defectivenozzles will follow only the physical offset and will thus be offset inthe resultant printed image 17. Thus, a simple calculation of thedifference between the images scanned by the image sensor with andwithout digital offset may be used as a reliable detection criterion fora failed nozzle.

A further exemplary embodiment of the calculation of the difference isthe integration of the image data recorded by the image sensor over aspecific period of time, of a short image section to be defined, tocreate a brightness profile. This process causes a defect in the printedimage to be immediately recognizable: if the nozzles work properly, thebrightness profile of the image data, potentially subdivided intoindividual channels, will match the brightness profile of the previousimage section 20. If distinctive maximum and minimum turning points inthe added-up brightness profile are offset in synchronism with theoffsetting of the printed image 16 relative to the nozzles, a defect 13caused by a defective or failed nozzle has occurred.

It is likewise possible to implement the digital offsetting of theprinted image without any mechanical counter-correction. In thiscontext, the printed image is offset by one or more printing nozzles ina direction transverse to the printing direction, causing differentnozzles to print the image information. However, no simultaneousmechanical counter-offset is implemented. In order to allow a correctcomparison 18, the image recorded by the camera is offset by therequired amount in the respective required direction. Alternatively, thecontrol unit may offset the scanned image 16 by the corresponding amountduring the evaluation of the image, i.e. prior to the calculation of thedifference. After this operation has been completed, the calculation ofthe difference of the successive images is possible in the same way.However, a disadvantage is that the absolute position on the substratechanges. For most print jobs, such a minimum absolute offset istolerable. It does not have any influence on the analysis and thecalculation of the difference anyway.

A further exemplary embodiment is a digital offset without mechanicalcounter-correction in a number of small steps: in this context, thetotal offset width is unknown or irrelevant as long as the printed imageis not moved out of the range of the available printing nozzles. Theimage analysis in the control unit recognizes the “optical flow” of themoving pixels and is able to differentiate between this and thestationary lines 13 that are created by failed nozzles. In this case, adisadvantage is that an “optical flow” is only created by multipleoffsetting processes made in small steps and thus requires a number ofprints. In addition, a disadvantage is that this process likewiseoffsets the absolute position on the substrate.

The method described above is illustrated in a flow chart shown in FIG.5. In a first step, a first printed image 12 produced according to printimage data is scanned by an image scanner. The first printed image 12 isthen digitally offset in the control unit 15 by at least one printingnozzle in a direction transverse to the printing direction. Furtherprint processing produces a second printed image 16 on the basis of thedigitally offset first printed image 12. The second printed image 16 isscanned and digitized by using the at least one image sensor. The firstand second scanned printed images 12, 16 are then compared in thecontrol unit 15. Finally, failed printing nozzles are identified byusing the control unit 15 based on a result 18 of the comparison and aresult 19 of a detection is output to an operator on a display by thecontrol unit 15. The detection result 19 is used as a trigger forstarting a compensation mode of the inkjet printing machine 1 tocompensate for the at least one failed printing nozzle.

The invention claimed is:
 1. A method for automated detection of failedprinting nozzles in an inkjet printing machine, the method comprisingthe following steps: carrying out a printing process to produce a firstprinted image based on print image data; scanning and digitizing thefirst printed image produced in the printing process by using at leastone image sensor; digitally offsetting the first printed image in acontrol unit by at least one printing nozzle in a direction transverseto a printing direction; carrying out a further printing process toproduce a second printed image based on the digitally offset firstprinted image; scanning and digitizing the second printed image by usingthe at least one image sensor; comparing the first and second scannedprinted images in the control unit; and identifying failed printingnozzles by using the control unit based on a result of the comparison.2. The method according to claim 1, which further comprises in additionto offsetting the first digital printed image by at least one printingnozzle in a direction transverse to the printing direction, offsetting aprint head by an equal amount in an opposite direction.
 3. The methodaccording to claim 1, which further comprises carrying out thecomparison between the first and second scanned digital printed imagesby calculating a difference.
 4. The method according to claim 1, whichfurther comprises outputting a detection result to an operator on adisplay by the control unit.
 5. The method according to claim 4, whichfurther comprises using the detection result as a trigger for starting acompensation mode of the inkjet printing machine to compensate for theat least one failed printing nozzle.
 6. The method according to claim 1,wherein the at least one image sensor only scans a respective imagesection of a respective printed image.
 7. The method according to claim6, which further comprises adding the scanned image sections during anongoing printing process to create a brightness profile, and calculatinga difference between brightness profiles of the scanned image sectionsof the respective first and second digital printed images.
 8. The methodaccording to claim 1, which further comprises offsetting the firstdigital printed image by at least two printing nozzles in the directiontransverse to the printing direction in steps of at least one printingnozzle.
 9. An inkjet printing machine, comprising: a control unit forautomated detection of failed printing nozzles in the inkjet printingmachine by performing the following steps: carrying out a printingprocess to produce a first printed image based on print image data;scanning and digitizing the first printed image produced in the printingprocess by using at least one image sensor; digitally offsetting thefirst printed image in a control unit by at least one printing nozzle ina direction transverse to a printing direction; carrying out a furtherprinting process to produce a second printed image based on thedigitally offset first printed image; scanning and digitizing the secondprinted image by using the at least one image sensor; comparing thefirst and second scanned printed images in the control unit; andidentifying failed printing nozzles by using the control unit based on aresult of the comparison.